The present invention is directed to a device for drilling undercuts in boreholes in a target material where the device includes a housing abutting against the surface of the target material with a guide sleeve containing an eccentrically arranged axially parallel bore in which an eccentric bushing having a handle is supported so that it can be rotated through an angle limited to approximately 180.degree.. A drive shaft, including a cutter head, coupled with a driving unit, is rotatably supported in the eccentric bushing.
Undercut sections in boreholes are formed mainly for receiving appropriately shaped anchor dowels so that the dowel can be anchored in a positive manner in the undercut. Because of the expensive and/or malfunction-prone devices for cutting the undercuts, such anchor dowels have not achieved notable success up to the present time.
In AT-PS 368 244, a device for forming undercuts is disclosed which is relatively difficult to handle and includes a guide sleeve, an eccentric bushing rotatably supported in the sleeve, and a drive shaft with a cutter head rotatably supported in the bushing. The eccentricity of the guide sleeve corresponds roughly to the eccentricity of the bushing. Accordingly, the drive shaft is located centrally in the bushing in one extreme position of the guide sleeve and the bushing. In the other extreme position, rotated through an angle of approximately 180.degree., the drive shaft is displaced radially outwardly by double the amount of eccentricity. Between these two extreme positions, the cutter head penetrates into the borehole surface. After the complete penetration, the eccentric bushing and the guide sleeve are rotated together through an angle of approximately 360.degree.. During such rotation, a circumferentially extending undercut is formed in the borehole surface. Subsequently, the eccentric bushing must be rotated in the guide sleeve through 180.degree. in either rotational direction for positioning drive shaft centrally in the guide sleeve so that it can be removed from the borehole. The guide sleeve and the eccentric bushing each have a separate handle for effecting rotation.
In EU-PA 0 231 720 there is another known device which is quite expensive in view of the improvement in the device. Such device functions in the same general manner as described above with the guide sleeve rotatably supported in an enclosing housing. The eccentric bushing is connected to a toothed ring, engageable with a worm shaft driven by a transmission. The eccentric bushing and the guide sleeve can be coupled together by an axially shiftable pin. After insertion of the device in a borehole, with the drive shaft arranged centrally, the eccentric bushing in the guide sleeve is rotated through 180.degree. by the transmission, until the shiftable pin rests against a stop in the guide sleeve. During subsequent further rotation of the transmission, the guide sleeve, along with the eccentric bushing, is rotated by the pin. After forming a complete undercut, the pin is shifted axially out of engagement, so that the eccentric bushing can be rotated further in the guide sleeve until the bore in the eccentric bushing is arranged centrally in the guide sleeve and the device can be removed from the borehole.